Cathode active material, cathode and nonaqueous secondary battery

ABSTRACT

A cathode active material of the present invention is a cathode active material having a composition represented by General Formula (1) below,
 
LiFe 1-x M x P 1-y Si y O 4   (1),
         where: an average valence of Fe is +2 or more; M is an element having a valence of +2 or more and is at least one type of element selected from the group consisting of Zr, Sn, Y, and Al; the valence of M is different from the average valence of Fe; 0&lt;x≦0.5; and y=x×({valence of M}−2)+(1−x)×({average valence of Fe}−2). This provides a cathode active material that not only excels in terms of safety and cost but also can provide a long-life battery.

This application is the U.S. national phase of International ApplicationNo. PCT/JP2010/058559 filed 20 May 2010 which designated the U.S. andclaims priority to JP 2009-124647 filed 22 May 2009, the entire contentsof each of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a cathode active material, a cathode inwhich such a cathode active material is used, and a nonaqueous secondarybattery (lithium secondary battery) in which such a cathode is used.More specifically, the present invention relates to a nonaqueoussecondary battery excellent in cycling characteristics.

BACKGROUND ART

Lithium secondary batteries have been in practical and widespread use assecondary batteries for portable electronic devices Furthermore, inrecent years, lithium secondary batteries have drawn attention not onlyas small-sized secondary batteries for portable electronic devices butalso as high-capacity devices for use in vehicles, power storage, etc.Therefore, there has been a growing demand for higher safety standards,lower costs, longer lives, etc.

A lithium secondary battery is composed mainly of a cathode, an anode,an electrolyte, a separator, and an armoring material. Further, thecathode is constituted by a cathode active material, a conductivematerial, a current collector, and a binder (binding agent).

In general, the cathode active material is realized by a layeredtransition metal oxide such as LiCoO₂. However, in a state of fullcharge, such layered transition metal oxides are prone to cause oxygendesorption at a comparatively low temperature of approximately 150° C.,and such oxygen desorption may cause a thermal runaway reaction in thebattery. Therefore, when a battery having such a cathode active materialis used for a portable electronic device, there is a risk of an accidentsuch as heating, firing, etc. of the battery.

For this reason, in terms of safety, expectations have been placed onlithium manganate (LiMn₂O₄) having a spinel-type structure, lithium ironphosphate (LiFePO₄) having an olivine-type structure, etc. that arestable in structure and do not emit oxygen in abnormal times.

Further, in terms of cost, cobalt (Co) is low in degree of existence inthe earth's crust and high in price. For this reason, expectations havebeen placed on lithium nickel oxide (LiNiO₂) or a solid solution thereof(Li(Co_(1-x)Ni_(x))O₂), lithium manganate (LiMn₂O₄), lithium ironphosphate (LiFePO₄), etc.

Further, in terms of life, the insertion and desorption of Li into andfrom a cathode active material along with charging and discharging causestructural destruction in the cathode active material. For this reason,more expectations have been placed on lithium manganate (LiMn₂O₄) havinga spinel-type structure, lithium iron phosphate (LiFePO₄) having anolivine-type structure, etc. than on layered transition metal oxidesbecause of their structural stability.

Therefore, for example, such lithium iron phosphate having anolivine-type structure has drawn attention as a cathode active materialfor a battery in consideration of safety, cost, and life. However, whenlithium iron phosphate having an olivine-type structure is used as acathode active material for a battery, there are such declines incharge-discharge behavior as insufficient electron conductivity and lowaverage potential.

In order to improve charge-discharge behavior, there has been proposedan active material represented by general formulaA_(a)M_(b)(XY₄)_(c)Z_(d) (where A is an alkali metal, M is a transitionmetal, XY₄ is PO₄ or the like, and 7 is OH or the like) (e.g., seePatent Literature 1).

Further, there have been also proposed an active material, representedby general formula LiMP_(1-x)A_(x)O₄ (where M is a transition metal, Ais an element having an oxidation number of +4 or less, and 0<X<1),whose P site has been replaced by the element A (e.g., see PatentLiterature 2).

Further proposed as a cathode active material for a nonaqueouselectrolyte secondary battery excellent in large-currentcharge-discharge behavior is a material represented by general formulaLi_(1-x)A_(x)Fe_(1-Y-Z)M_(y)Me_(z)P_(1-m)X_(m)O_(4-n)Z_(n) (where A isNa or K; M is a metal element other than Fe, Li, and Al; X is Si, N, orAs; Z is F, Cl, Br, I, S, or N) (e.g., see Patent Literature 3). Furtherproposed as an electrode active material that can be economicallyproduced, is satisfactory in charging capacity, and is satisfactory inrechargeability over many cycles is a material represented by generalformula A_(a+x)M_(b)P_(1-x)Si_(x)O₄ (where A is Ki or Na, or K; and M isa metal) (e.g., see Patent Literature 4).

There has also been disclosed lithium transition metal phosphorus, suchas LiFePO₄, which includes at least two coexisting phases including alithium-rich transition metal phosphate phase and a lithium-poortransition metal phosphate phase, the coexisting phases being differentfrom each other in molar volume by approximately 5.69 (e.g., see Table 2of Patent Literature 5).

CITATION LIST Patent Literature 1

-   Japanese Patent Application Publication (Translation of PCT    Application), Tokuhyou, No. 2005-522009 (Publication Date: Jul. 21,    2005)

Patent Literature 2

-   Japanese Patent Application Publication (Translation of PCT    Application), Tokuhyou, No. 2008-506243 (Publication Date: Feb. 28,    2008)

Patent Literature 3

-   Japanese Patent Application Publication, Tokukai, No. 2002-198050 A    (Publication Date: Jul. 12, 2002)

Patent Literature 4

-   Japanese Patent Application Publication (Translation of PCT    Application), Tokuhyou, No. 2005-519451 (Publication Date: Jun. 30,    2005)

Patent Literature 5

-   PCT International Publication No. 2008/039170, pamphlet (Publication    Date: Apr. 3, 2008)

SUMMARY OF INVENTION Technical Problem

Unfortunately, however, the active materials structured as described inPatent Literatures 1 to 5 above result in short-life batteries.

Specifically, according to the structures of the active materials asdescribed in Patent Literatures 1 to 5, the insertion and desorption ofLi into and from a cathode active material along with charging anddischarging cause great expansion or contraction in the cathode activematerial; therefore, an increase in the number of cycles may cause thecathode active material to gradually detach from the current collectorand the conductive material physically and therefore cause structuraldestruction in the cathode active material. This is because a materialthat greatly expands or contracts due to charging and discharging causesdestruction of secondary particles and destruction of the conductivepath between the cathode active material and the conductive material andtherefore causes an increase in internal resistance of the battery. Thisresults in an increase in active materials that do not contribute tocharging or discharging, causes a decrease in capacity, and thereforemakes the battery short lived.

As mentioned above, there has been a demand for cathode active materialsexcellent in terms of safety, cost, and life. However, the activematerials structured as described in Patent Literatures 1 and 2 aboveare high in rate of expansion and contraction in volume (rate of changein volume) during charging and discharging and therefore result in shortlives.

The present invention has been made in view of the foregoing problems,and it is an object of the present invention to realize a cathode activematerial that not only excels in terms of safety and cost but also canprovide a long-life battery, a cathode in which such a cathode activematerial is used, and a nonaqueous secondary battery in which such acathode is used.

Solution to Problem

The present invention extends the life of a battery through suppressionof expansion and contraction by carrying out element substitution withuse of lithium iron phosphate as a basic structure.

Specifically, in order to solve the above problems, a cathode activematerial of the present invention is a cathode active material having acomposition represented by General Formula (1) below,LiFe_(1-x)M_(x)P_(1-y)Si_(y)O₄  (1),

where: an average valence of Fe is +2 or more; M is an element having avalence of +2 or more and is at least one type of element selected fromthe group consisting of Zr, Sn, Y, and Al; the valence of M is differentfrom the average valence of Fe; 0<x≦0.5; and y=x×({valence ofM}−2)+(1−x)×({average valence of Fe}−2).

According to the foregoing structure, a change in volume during Liinsertion and desorption can be suppressed by replacing at least part ofP site with Si and replacing part of Fe site with an element capable ofcompensation for charges in the crystal structure. As a result, in thecase of a battery made with use of such a cathode active material, thecathode can be prevented from expanding or contracting due to chargingand discharging. This brings about an effect of providing a cathodeactive material that not only excels in terms of safety and cost butalso can provide a long-life battery.

Furthermore, Zr, Sn, Y, and Al are easily combined because they do notchange in valence, can be combined in a reducing atmosphere, and do notrequire control of the partial pressure of oxygen for controlling thevalence of a substituting element.

In order to solve the foregoing problems, a cathode of the presentinvention includes: the cathode active material of the presentinvention; a conductive material; and a binder.

According to the foregoing structure, the inclusion of such a cathodeactive material according to the present invention brings about aneffect of providing a cathode that not only excels in terms of safetyand cost but also can provide a long-life battery.

In order to solve the foregoing problems, a nonaqueous secondary batteryof the present invention includes: the cathode of the present invention;an anode; an electrolyte; and a separator.

According to the foregoing structure, the inclusion of such a cathodeaccording to the present invention brings about an effect of providing along-life battery excellent in terms of safety and cost.

A module of the present invention includes a combination of a pluralityof the nonaqueous secondary battery of the present invention.

According to the foregoing structure, the inclusion of such a nonaqueoussecondary battery according to the present invention brings about aneffect of providing a long-life module excellent in terms of safety andcost.

A power storage system of the present invention includes the nonaqueoussecondary battery of the present invention.

According to the foregoing structure, the inclusion of such a nonaqueoussecondary battery according to the present invention brings about aneffect of providing a long-life power storage system excellent in termsof safety and cost.

Advantageous Effects of Invention

As described above, a cathode active material of the present inventionhas a composition represented by General Formula (1) above.

This brings about an effect of providing a cathode active material thatnot only excels in terms of safety and cost but also can provide along-life battery.

As described above, a cathode of the present invention includes: thecathode active material of the present invention; a conductive material;and a binder.

This brings about an effect of providing a cathode that not only excelsin terms of safety and cost but also can provide a long-life battery.

As described above, a nonaqueous secondary battery of the presentinvention includes: the cathode of the present invention; an anode; anelectrolyte; and a separator.

This brings about an effect of providing a long-life battery excellentin terms of safety and cost.

A module of the present invention includes a combination of a pluralityof the nonaqueous secondary battery of the present invention.

This brings about an effect of providing a long-life module excellent interms of safety and cost.

A power storage system of the present invention includes the nonaqueoussecondary battery of the present invention.

This brings about an effect of providing a long-life power storagesystem excellent in terms of safety and cost.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph illustrating an X-ray diffraction pattern for acathode active material prepared in Example 2.

FIG. 2 is a graph illustrating an X-ray diffraction pattern for acathode active material prepared in Example 3.

FIG. 3 is a graph illustrating an X-ray diffraction pattern for acathode active material prepared in Example 4.

FIG. 4 is a graph illustrating an X-ray diffraction pattern for acathode active material prepared in Example 5.

FIG. 5 is a graph illustrating an X-ray diffraction pattern for acathode active material prepared in Example 6.

FIG. 6 is a graph illustrating an X-ray diffraction pattern for acathode active material prepared in Example 7.

FIG. 7 is a graph illustrating an X-ray diffraction pattern for acathode active material prepared in Comparative Example 1.

FIG. 8 is a graph illustrating an absorption spectrum measured byMössbauer spectrometry of the cathode active material prepared inExample 2.

FIG. 9 is a graph illustrating an absorption spectrum measured byMössbauer spectrometry of the cathode active material prepared inExample 3.

FIG. 10 is a cross-sectional view schematically illustrating aconfiguration of a battery used for evaluation of a capacity retentionrate in Examples.

FIG. 11 is a perspective view schematically illustrating a configurationof a flat-plate laminate battery prepared in Example 9.

FIG. 12 is a perspective view schematically illustrating a configurationof a layered cuboidal battery prepared in Example 10.

FIG. 13 is a perspective view schematically illustrating a configurationof a wound cylindrical battery prepared in Example 11.

DESCRIPTION OF EMBODIMENTS

The present invention is described below in detail. It should be noted,in this specification, that the range “A to B” means “A or more to B orless”. Further, the various properties enumerated in this specificationmean values measured by methods described later in Examples, unlessotherwise noted.

(I) Cathode Active Material

A cathode active material of the present embodiment is a cathode activematerial having a composition represented by General Formula (1) below,LiFe_(1-x)M_(x)P_(1-y)Si_(y)O₄  (1),

where: an average valence of Fe is +2 or more; M is an element having avalence of +2 or more and is at least one type of element selected fromthe group consisting of Zr, Sn, Y, and Al; the valence of M is differentfrom the average valence of Fe; 0<x≦0.5; and y=x×({valence ofM}−2)+(1−x)×({average valence of Fe}−2).

The term “average valence of Fe” here means the average of the valencesof all the Fe atoms constituting the cathode active material.

In general, in the case of olivine-type lithium iron phosphate, there isa contraction in volume during desorption of Li from the initialstructure due to charging. During this structural change, there arecontractions along the a axis and the b axis, and there is an expansionalong the c axis. For this reason, the inventors found it possible tosuppress a change in volume by reducing the rates of contraction alongthe a axis and the b axis and increasing the rate of expansion along thec axis through any sort of substitution.

Then, the inventors found that by replacing part of P site with Si andreplacing part of Fe site with another atom, compensation for charges inthe crystal structure is made and a change in volume during Lidesorption is suppressed, whereby expansion and contraction due tocharging and discharging are also suppressed.

It should be noted that although most of the materials that havecompositions represented by general formula (1) have olivine-typestructures, the scope of the present invention is not limited to thosematerials which have olivine-type structures. Those materials which donot have olivine-type structures are also encompassed in the scope ofthe present invention.

In the cathode active material according to the present embodiment, Psite has been replaced by Si, and P and Si are different in valence fromeach other. Therefore, it is necessary to make compensation for chargesin the crystal structure. For this reason, Fe site has been replaced byM.

That is, because the valences of P and Si in general formula (1) are +5and +4, respectively, the substitution amount y of Si comes to satisfyy=x×({valence of M}−2)+(1−x)×({average valence of Fe}−2) according tothe principle that the total of charges in the structure is 0.

In General Formula (1), y preferably falls within the range (x×({valenceof M}−2))≦y<(x×({valence of M}−2)+0.05).

Fe in general formula (1) can generally take on a valence of +2 or +3.Fe²⁺ is preferably contained at a proportion of not less than 95% ascalculated from a Mössbauer spectrum. More preferably, the averagevalence of Fe is +2, and particularly preferably, every Fe has a valenceof +2.

In the present embodiment, the rate of change in volume of a unit cellin Li_(x)Fe_(1-x)M_(x)P_(1-y)Si_(y)O₄ is preferably 5% or less, or morepreferably 4% or less, relative to the volume of a unit cell in GeneralFormula (1).

The reason why the rate of change in volume is preferably 4% or less isthat the cathode active material according to the present embodiment hasa change in slope of the volume maintenance ratio relative to the rateof change in volume at a point where the rate of change in volume (rateof expansion and contraction due to charging and discharging) of thevolume of a unit cell reaches approximately 4%. That is, when the rateof change in volume becomes higher than approximately 4%, the volumemaintenance ratio comes to decrease to a greater extent than the rate ofchange in volume increases. Therefore, if the rate of change in volumeis approximately 4% or less, it is possible to better suppress adecrease in volume maintenance ratio.

The element M, which replaces Fe site, is an element capable of takingon a valence of +2 or more and at least one type of element selectedfrom the group consisting of Zr, Sn, Y, and Al. Further, it ispreferable that the element M, which replaces Fe site, be an elementhaving a valence of +3 or +4. For a greater effect of suppressing therate of change in volume, it is more preferable that Fe site be replacedby an element having a valence of +4.

It is preferable that the trivalent element M, which replaces Fe site,be Y, because Y does not change in valence during synthesis. Since thereoccurs no change in valence during synthesis, the cathode activematerial can be synthesized stably.

It is preferable that the tetravalent element M, which replaces Fe site,be Zr or Sn, because Zr and Sn do not change in valence duringsynthesis. Since there occurs no change in valence during synthesis, thecathode active material can be synthesized stably.

It is preferable that M in general formula (1) have a valence of +3 or+4, and it is more preferable that every M have a valence of +3 or thatevery M have a valence of +4.

The substitution amount x on Fe site falls within a range of larger than0 to 0.5 or smaller. If the substitution amount x on Fe site fallswithin the above range, it is possible to prevent (i) a significantreduction in the discharging capacity of a battery in which the cathodeactive material is used and (ii) a volume change occurring during Liinsertion and desorption.

The larger the amount of substitution on Fe site is, the better the rateof change in volume can be suppressed. In other words, the larger theamount of substitution on Fe site is, the better the volume maintenanceratio is at 500 cycles. If the rate of change in volume is 4% or less,the volume maintenance ratio can be 90% or more.

On the other hand, the larger the amount of substitution on Fe site, thesmaller the initial capacity is. In the case where Fe is replaced by Zr,the substitution amount x on Fe site is (i) preferably 0.35 or less toobtain an initial capacity of 100 mAh/g or greater, (ii) more preferably0.3 or less to obtain an initial capacity of 110 mAh/g or greater, or(iii) even more preferably 0.25 or less to obtain an initial capacity of120 mAh/g or greater.

In the case where Fe is replaced by Sn, the substitution amount x on Fesite is (i) preferably 0.3 or less to obtain an initial capacity of 100mAh/g or greater, (ii) more preferably 0.25 or less to obtain an initialcapacity of 110 mAh/g or greater, or (iii) even more preferably 0.2 orless to obtain an initial capacity of 120 mAh/g or greater.

In the case where Fe is replaced by Y, the substitution amount x on Fesite is (i) preferably 0.35 or less to obtain an initial capacity of 100mAh/g or greater, (ii) more preferably 0.3 or less to obtain an initialcapacity of 110 mAh/g or greater, or (iii) even more preferably 0.25 orless to obtain an initial capacity of 120 mAh/g or greater.

In the case where Fe is replaced by Al, the substitution amount x on Fesite is (i) preferably 0.45 or less to obtain an initial capacity of 100mAh/g or greater, more preferably 0.4 or less to obtain an initialcapacity of 110 mAh/g or greater, or (iii) even more preferably 0.3 orless to obtain an initial capacity of 120 mAh/g or greater.

When Fe site is replaced by metal atoms having a valence of +3 and everyFe has a valence of +2, the same amount of Si as the amount ofsubstitution of Fe site is required for the maintenance ofelectroneutrality. In this case, the amount of substitution ispreferably 0.25 or greater for Al and 0.15 or greater for Y to keep therate of change in volume to 5% or less. Further, the amount ofsubstitution is preferably 0.35 or greater for Al and 0.2 or greater forY to keep the rate of change in volume to 4% or less.

When Fe site is replaced by metal atoms having a valence of +4 and everyFe has a valence of +2, the amount of Si twice as large as the amount ofsubstitution of Fe site is required for the maintenance ofelectroneutrality. In this case, the amount of substitution ispreferably 0.05 or greater for Zr and 0.15 or greater for Sn to keep therate of change in volume to 5% or less. The amount of substitution ispreferably 0.15 or greater for Zr and 0.25 or greater for Sn to keep therate of change in volume to 4% or less. The amount of substitution ispreferably 0.2 or greater for Zr and 0.3 or greater for Sn to keep therate of change in volume to 3% or less. Further, the amount ofsubstitution is preferably 0.25 or greater for Zr to keep the rate ofchange in volume to 2% or less.

The present invention encompasses the following embodiment: When Fe siteis replaced by Zr atoms having a valence of +4 and every Fe has avalence of +2, the substitution amount x of Zr may be within the range0.05≦x≦0.15.

The aforementioned cathode active material according to the presentembodiment can be produced by using any combination of a carbonate ofeach element, a hydroxide of each element, a chloride salt of eachelement, a sulfate salt of each element, an acetate salt of eachelement, an oxide of each element, an oxalate of each element, a nitratesalt of each element, etc. as raw materials. Examples of productionmethods include methods such as a solid-phase method, a sol-gel process,melt extraction, a mechanochemical method, a coprecipitation method, ahydrothermal method, evaporative decomposition, etc. Further, as hasbeen commonly done in olivine-type lithium iron phosphate, electricalconductivity may be improved by covering the cathode active materialwith a carbon film.

As described above, the cathode active material of the present inventionmay preferably be arranged such that a rate of change in volume of aunit cell in Li_(x)Fe_(1-x)M_(x)P_(1-y)Si_(y)O₄ is 5% or less withrespect to a volume of a unit cell in General Formula (1).

According to the foregoing structure, the rate of change in volume is 5%or less. This makes it possible to better prevent a cathode fromexpanding or contracting due to charging and discharging, thus making itpossible to provide a cathode active material capable of providing along-life battery.

The cathode active material of the present invention may preferably bearranged such that a rate of change in volume of a unit cell inLi_(x)Fe_(1-x)M_(x)P_(1-y)Si_(y)O₄ is 4% or less with respect to avolume of a unit cell in General Formula (1).

According to the foregoing structure, the rate of change in volume is 4%or less. This makes it possible to better prevent a cathode fromexpanding or contracting due to charging and discharging, thus making itpossible to provide a cathode active material capable of providing along-life battery.

The cathode active material of the present invention may preferably bearranged such that the valence of M is +4.

The foregoing structure, which is highly effective in suppressing therate of change in volume, it possible to better prevent the cathode fromexpanding or contracting due to charging and discharging, thus making itpossible to provide a cathode active material capable of providing along-life battery.

The cathode active material of the present invention may preferably bearranged such that M in General Formula (1) is Zr; and 0.05≦x≦0.5.

According to the foregoing structure, the rate of change in volume isapproximately 5% or less. This makes it possible to better prevent acathode from expanding or contracting due to charging and discharging,thus making it possible to provide a cathode active material capable ofproviding a long-life battery.

The cathode active material of the present invention may preferably bearranged such that M in General Formula (1) is Zr; and 0.15≦x≦0.5.

According to the foregoing structure, the rate of change in volume isapproximately 4% or less. This makes it possible to better prevent acathode from expanding or contracting due to charging and discharging,thus making it possible to provide a cathode active material capable ofproviding a long-life battery.

The cathode active material of the present invention may preferably bearranged such that M in General Formula (1) is Zr; and 0.25≦x≦0.5.

According to the foregoing structure, the rate of change in volume isapproximately 2% or less. This makes it possible to even better preventa cathode from expanding or contracting due to charging and discharging,thus making it possible to provide a cathode active material capable ofproviding a battery with an even long life.

The cathode active material of the present invention may preferably bearranged such that M in General Formula (1) is Sn; and 0.25≦x≦0.5.

According to the foregoing structure, the rate of change in volume isapproximately 4% or less. This makes it possible to better prevent acathode from expanding or contracting due to charging and discharging,thus making it possible to provide a cathode active material capable ofproviding a long-life battery.

The cathode active material of the present invention may preferably bearranged such that the valence of M in General Formula (1) is +3.

The cathode active material of the present invention may preferably bearranged such that M in General Formula (1) is Y; and 0.2≦x≦0.5.

According to the foregoing structure, the rate of change in volume isapproximately 4% or less. This makes it possible to better prevent acathode from expanding or contracting due to charging and discharging,thus making it possible to provide a cathode active material capable ofproviding a long-life battery. Further, because Y does not change invalence during synthesis of a cathode active material, the cathodeactive material can be synthesized stably.

The cathode active material of the present invention may preferably bearranged such that M in General Formula (1) is Al; and 0.35≦x≦0.5.

According to the foregoing structure, the rate of change in volume isapproximately 4% or less. This makes it possible to better prevent acathode from expanding or contracting due to charging and discharging,thus making it possible to provide a cathode active material capable ofproviding a long-life battery.

The cathode active material of the present invention may preferably bearranged such that the average valence of Fe in General Formula (1) is+2.

The foregoing structure makes it possible to better prevent the cathodefrom expanding or contracting due to charging and discharging, thusmaking it possible to provide a cathode active material capable ofproviding a long-life battery.

The cathode active material of the present invention may preferably bearranged such that M in General Formula (1) is Zr; and 0.05≦x≦0.15.

The foregoing structure makes it possible to better prevent the cathodefrom expanding or contracting due to charging and discharging, thusmaking it possible to provide a cathode active material capable ofproviding a long-life battery.

(II) Nonaqueous Secondary Battery

A nonaqueous secondary battery according to the present embodiment has acathode, an anode, an electrolyte, and a separator. Each of thecomponents is described below. It should be noted that it is preferablethat the nonaqueous secondary battery according to the presentembodiment be a laminate battery, a layered cuboidal battery, a woundcuboidal battery, or a wound cylindrical battery.

(a) Cathode

The cathode, composed of such a cathode active material according to thepresent embodiment, a conductive material, and a binder, can be made,for example, by a publicly-known method such as application to a currentcollector of a slurry obtained by mixing the active material, theconductive material, and the binder with an organic solvent.

Usable examples of the binder (binding agent) includepolytetrafluoroethylene, polyvinylidene fluoride, polyvinylchloride,ethylene-propylene diene polymer, styrene-butadiene rubber,acrylonitrile-butadiene rubber, fluorocarbon rubber, polyvinyl acetate,polymethyl methacrylate, polyethylene, nitrocellulose, etc.

Usable examples of the conductive material include acetylene black,carbon, graphite, natural graphite, artificial graphite, needle coke,etc.

Usable examples of the current collector include a foam (porous) metalhaving continuous holes, a metal shaped in a honeycomb pattern, asintered metal, an expanded metal, nonwoven cloth, a plate, foil, aperforated plate, perforated foil, etc.

Usable examples of the organic solvent include N-methylpyrrolidone,toluene, cyclohexane, dimethylformamide, dimethylacetoamide, methylethyl ketone, methyl acetate, methyl acrylate, diethyltriamine,N—N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran, etc.

It is preferable that the cathode have a thickness of approximately 0.01to 20 mm. Too great a thickness undesirably causes a decrease inelectrical conductivity, and too small a thickness undesirably causes adecrease in capacity par unit area. It should be noted that the cathode,obtained by application and drying, may be consolidated by a rollerpress, etc. so that the active material has a higher filling density.

(b) Anode

The anode can be made by a publicly-known method. Specifically, theanode can be made by the same method as described in the method formaking the cathode, i.e., by mixing such a publicly-known binding agentand such a publicly-known conductive material as named in the method formaking the cathode with an anodic active material, molding the mixedpowder into a sheet, and then pressure-bonding the molded product to anet (current collector) made of a conducting material such as stainlesssteel or copper. Alternatively, the anodic can also be made by applying,onto a substrate made of metal such as copper, a slurry obtained bymixing the mixed powder with such a publicly-known organic solvent asnamed in the method for making the cathode.

The anodic active material may be a publicly-known material. In order toconstitute a high-energy density battery, it is preferable that thepotential of insertion/desorption of lithium be close to thedeposition/dissolution potential of metal lithium. Typical examples ofsuch an anodic active material include carbon materials such as naturalor artificial graphite in the form of particles (scales, clumps, fibers,whisker, spheres, crushed particles, etc.).

Examples of the artificial graphite include graphite obtainable bygraphitizing mesocarbon microbeads, mesophase pitch powder, isotropicpitch powder, etc. Alternatively, it is possible to use graphiteparticles having amorphous carbon adhering to their surfaces. Amongthese, natural graphite is more preferable because it is inexpensive,close in oxidation-reduction potential to lithium, and can constitute ahigh-energy density battery.

Alternatively, it is possible to use a lithium transition metal oxide, atransition metal oxide, oxide silicon, etc. as the anodic activematerial. Among these, Li₄Ti₅O₁₂ is more preferable because it is highin potential flatness and small in volume change due to charging anddischarging.

(c) Electrolyte

Usable examples of the electrolyte include an organic electrolyte, a gelelectrolyte, a polymer solid electrolyte, an inorganic solidelectrolyte, a molten salt, etc. After injection of the electrolyte, anopening in the battery is sealed. It is possible to turn on electricitybefore the sealing and remove gas generated.

Examples of an organic solvent that constitutes the organic electrolyteinclude: cyclic carbonates such as propylene carbonate (PC), ethylenecarbonate (EC), and butylene carbonate; chain carbonates such asdimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methylcarbonate, and dipropyl carbonate; lactones such as γ-butyrolactone(GBL), γ-Valerolactone; furans such as tetrahydrofuran and 2-methyltetrahydrofuran; ethers such as diethyl ether, 1,2-dimethoxy ethane,1,2-diethoxy ethane, ethoxy methoxy ethane, dioxane; dimethyl sulfoxide;sulforan; methyl sulforan; acetonitrile; methyl formate; methyl acetate;etc. These organic solvents can be used alone or in combination of twoor more of them.

Further, the cyclic carbonates such as PC, EC, and butylene carbonateare high boiling point solvents and, as such, are suitable as a solventto be mixed with GBL.

Examples of an electrolyte salt that constitutes the organic electrolyteinclude lithium salts such as fluoroboric lithium (LiBF₄), lithiumhexafluorophosphate (LiPF₆), trifluoromethanesulfonic lithium(LiCF₃SO₃), trifluoroacetic lithium (LiCF₃COO), trifluoraceticbis(trifluoromethanesulfone)imide (LiN(CF₃SO₂)₂), etc. These electrolytesalts can be used alone or in combination of two or more of them. Asuitable salt concentration of the electrolyte is 0.5 to 3 mol/l.

(d) Separator

Examples of the separator include a porous material, nonwoven cloth,etc. It is preferable that the separator be made of such a material asmentioned above that neither dissolves not swells in response to theorganic solvent contained in the electrolyte. Specific examples arepolyester polymers, polyolefin polymers (e.g., polyethylene,polypropylene), ether polymers, and inorganic materials such glass, etc.

The components, such as the separator, a battery case, and otherstructural materials, of the battery according to the present embodimentmay be, but are not particularly limited to, various materials that areused in a conventional publicly-known nonaqueous secondary battery.

(e) Method for Producing a Nonaqueous Secondary Battery

The nonaqueous secondary battery according to the present embodiment canbe made, for example, by layering the cathode and the anodic in such away that the separator is sandwiched between them. The layered electrodemay have a rectangular planar shape. Further, when a cylindrical or flatbattery is made, the layered electrode may be wound.

Such a single layered electrode or a plurality of such layeredelectrodes is/are inserted into a battery container. Usually, thecathode(s) and the anodic(s) are each connected to an externalconductive terminal of the battery. After that, the battery container issealed so that the electrode(s) and the separator(s) are shielded fromoutside air.

In the case of a cylindrical battery, the battery container is usuallysealed by fitting a resin gasket in the opening of the battery containerand then caulking the battery container. In the case of a cuboidalbattery, the battery container can be sealed by mounting a metal lid(called a sealing plate) on the opening and then joining them bywelding. Other than these methods, the battery container can be sealedby a binding agent or by fastening it with a bolt through a gasket.Furthermore, the battery container can be sealed by a laminate filmobtained by joining a thermoplastic resin on top of metal foil. Whensealed, the battery container may be provided with an opening throughwhich the electrolyte is injected.

As described above, the cathode active material according to the presentinvention undergoes only a small change in volume duringcharging/discharging, and is thus less likely to cause destruction ofsecondary particles or destruction of the conductive path between thecathode active material and the conductive material. Therefore, thecathode active material itself has a long life.

An electrode prepared by applying a conventional cathode active materialonto a metal foil made of, for example, aluminum is, since the cathodeactive material has a large change in volume duringcharging/discharging, problematic in that the thickness of the electrodeitself changes during charging/discharging.

If the thickness of the electrode itself changes, a battery armorcontaining a collection of such electrodes is repeatedly subjected tostress. In the case where the battery armor is made of a metal, suchrepeated stress may cause a crack in the battery armor itself or asealing part. In the case where the battery armor is made of, forexample, a laminated material, repeated stress causes only a littlefatigue, but changes the thickness of the battery itself, which in turncauses stress to a module containing such batteries stacked on oneanother. This may decrease reliability of the module.

In contrast, an electrode prepared by applying the cathode activematerial of the present invention onto a metal foil made of, forexample, aluminum, since the cathode active material has only a smallchange in volume during charging/discharging, has a small change inthickness during charging/discharging. This reduces a change in thethickness of the battery during charging/discharging, and thus reducesstress on the armor of the battery in the case where the armor is madeof a metal. As a result, it is possible to provide a highly reliablebattery.

A battery including the cathode of the present invention, as describedabove, excels in long-term reliability, and is thus suitably used tostore power over an extended period of time, the power including solarpower, late-night power, and power from a natural energy such as windpower, geothermal power, and wave power.

The present invention is not limited to the description of theembodiments above, but may be altered by a skilled person within thescope of the claims. An embodiment based on a proper combination oftechnical means disclosed in different embodiments is encompassed in thetechnical scope of the present invention.

EXAMPLES

The present invention is described below in more detail with referenceto Examples; however, the present invention is not limited to Examplesbelow. It should be noted that reagents etc. used in Examples arehighest quality reagents manufactured by Kishida Chemical Co., Ltd.

(I) Calculation of Rate of Change in Volume and Theoretical DischargingCapacity and Evaluation of Calculation Results

[References 1 to 4]

For each of the compounds listed in Table 1, the rate of change involume of the compound (the rate of change in volume of a unit cell inLi_(x)Fe_(1-x)M_(x)P_(1-y)Si_(y)O₄ relative to the volume of a unit cellin general formula (1)) was calculated according to the VASP, which is ageneral program for first principle calculation.

Specifically, the volume of a unit cell having four Li atoms, four Featoms, four P atoms, and sixteen O atoms was calculated under thefollowing conditions: ENCUT=400, IBRION=1, ISIF=3, EDIFF=1.0e-05,EDIFFG=−0.02, ISPIN=2. Further, the value U of Fe was 3.71.

The rate of change in volume was calculated according to the followingformula:Rate of change in volume(%)=(V ₀ −V ₁)/V ₀×100,where V₀ is the volume as calculated in the presence of Li; and V₁ isthe volume as calculated in the absence of Li.

For consideration of the amounts of substitution, calculations wereperformed on structures twice and four times as large as a unit cell,with half and a quarter the amount of substitution of each element. Thecalculations confirmed that the amount of substitution was directlyproportional to a lattice constant. The rate of change in volume foreach amount of substitution was calculated in a similar manner.

Further, from (i) the amount of change in valence of Fe from +2 to +3during discharging and (ii) the molecular weight of the compound, atheoretical discharging capacity of the compound was calculated.Specifically, the theoretical discharging capacity was calculatedaccording to the following formula:Theoretical discharging capacity(mAh/g)=F/3600/Mw×1000×(1−x),where F is a Faraday constant; Mw is the molecular weight of thecompound; and x, which is equivalent to x in General Formula (1), is theamount of substitution by M of Fe site.

Table 1 shows the results of the above calculation.

It should be noted that among values that are calculated according tofirst principle calculation, such a rate of change in volume iscalculated with high reproducibility because the lattice constant is avalue that contains few errors in calculation. These calculation resultscoincide highly accurately with values obtained by actually preparingcathode active materials and measuring their rates of change in volume.

The above calculation of the theoretical discharging capacity uses ageneral formula for calculating a theoretical capacity, and thus uses achange in valence of a transition metal element from +2 to +3. Thecalculation gives a maximum value of the capacity of an actuallysynthesized material. As will be described in Reference 5 below, lithiumiron phosphate with no substitution achieved a capacity substantiallyequivalent to the theoretical capacity. These calculation results shouldcoincide highly accurately with values obtained by actually preparingbatteries with use of cathode active materials and measuring theirdischarging capacities.

TABLE 1 Value of x Ref 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.501 Li(Fe_(1-x)Zr_(x))(P_(1-2x)Si_(2x))O₄ Rate of change in 5.84 4.65 3.462.26 1.05 −0.17 −1.39 volume (%) Theoretical dis- 159.8 149.8 140.1130.6 121.2 112.0 103.0 charging capacity (mAh/g) 2Li(Fe_(1-x)Sn_(x))(P_(1-2x)Si_(2x))O₄ Rate of change 6.35 5.68 5.00 4.333.65 2.98 2.30 in volume (%) Theoretical dis- 158.4 147.3 136.6 126.3116.4 106.8 97.5 charging capacity (mAh/g) 3Li(Fe_(1-x)Y_(x))(P_(1-x)Si_(x))O₄ Rate of change 6.16 5.31 4.46 3.612.76 1.92 1.08 0.24 in volume (%) Theoretical dis- 159.9 150.1 140.4130.9 121.6 112.5 103.5 94.7 charging capacity (mAh/g) 4Li(Fe_(1-x)Al_(x))(P_(1-x)Si_(x))O₄ Rate of change 6.59 6.16 5.73 5.294.85 4.41 3.97 3.52 3.08 2.63 in volume (%) Theoretical dis- 163.1 156.1148.9 141.6 134.2 126.6 118.8 110.9 102.8 94.5 charging capacity (mAh/g)

As shown in Table 1, each of the compounds of References 1 to 4exhibited a low rate of change in volume without significantly reducingits theoretical discharging capacity. This low rate of change in volumemeans that each of the compounds of References 1 to 4 had a low rate ofchange in volume during charging and discharging, and was therefore acathode active material with which a long-life battery can be produced.

[Reference 5]

The accuracy of the calculation results was confirmed by actuallypreparing cathode active materials from LiFePO₄ and FePO₄, respectively,and calculating their rates of change in volume. Table 2 shows theresults.

<Synthesis of LiFePO₄>

A lithium source LiOH, an iron source Fe(CH₃COO)₂, and a phosphatesource H₃PO₄ were used as starting materials, and these startingmaterials were measured out so that the molar ratio was Li:Fe:P=1:1:1.Next, the Fe source and the P source were put into a small amount ofwater, and the Li source was put after the Fe source had been completelydissolved. Into this aqueous solution, sucrose containing 20 percent bymass of LiFePO₄, which would be a final product, was added. This aqueoussolution was dried overnight at 60° C. in a drying furnace under anitrogen flow, and then sintered for twelve hours at 600° C. Thussynthesized was LiFePO₄ single-phase powder, which is an olivine-typecathode active material.

<Measurement of the Rate of Change in Volume>

The LiFePO₄ cathode active material thus synthesized was crushed in amortar into fine powder, and the lattice constant was calculated byX-ray measurement at 10° to 90° at room temperature with use of a Cutube.

Further, the lattice constant of an active material after desorption ofLi was calculated by using, as a cathode active material after Lidesorption, a cathode active material whose charging capacity had beenconfirmed and which had the same composition as in a state of Lidesorption and performing X-ray measurement on the cathode activematerial at room temperature. Specifically, XRD measurement of thecathode active material after Li desorption was performed afterpreparing a battery according to the after-mentioned method forpreparing a battery, taking out the cathode with the battery fullycharged, and then washing the cathode with ethanol.

After calculating the volume of a structure during charging and thevolume of the structure during discharging according to the latticeconstant of the structure during charging and the lattice constant ofthe structure during discharging, the rate of change in volume (%) dueto charging and discharging was calculated according to the followingformula:Rate of change in volume(%)=(1−volume of structure duringcharging/volume of structure during discharging)×100.

It should be noted here that the structure during charging is astructure during Li desorption and the structure during discharging isan initial structure during synthesis.

<Method for Preparing a Battery>

After the cathode active material, acetylene black (marketed as “DenkaBlack”; manufactured by Denki Kagaku Kogyo Kabushiki Kaisha), and PVdF(polyvinylidene fluoride) (marketed as “KF Polymer”; manufactured byKureha Corporation) were mixed with a mass ratio of 70:30:10, themixture was mixed with N-methylpyrrolidone (manufactured by KishidaChemical Co., Ltd.) to form slurry. A cathode was obtained by applyingthe slurry onto a 20-μm-thick aluminum foil so that the cathode had athickness of 50 μm to 100 μm. It should be noted that the cathode had anelectrode size of 2 cm×2 cm.

After the cathode had been dried, the cathode was used as an electrodeand Li metal was used as a counter electrode, with 50 ml of anelectrolyte contained in a 100-ml glass container. The electrolyte(manufactured by Kishida Chemical Co., Ltd.) used was obtained bydissolving LiPF₆ in a solvent so that the concentration was 1.4 mol/l,and the solvent used was obtained by mixing ethylene carbonate anddiethyl carbonate with a volume ratio of 7:3.

The battery prepared as above was charged and discharged at a rate of0.1 C, which showed that the battery had a capacity of 163 mAh/g.

TABLE 2 Experimental Calculated Composition Item value value LiFePO₄ aaxis (angstrom) 10.33 10.207 b axis (angstrom) 6.01 5.978 c axis(angstrom) 4.69 4.666 Volume (angstrom³) 291.17 284.71 FePO₄ a axis(angstrom) 9.82 9.753 b axis (angstrom) 5.79 5.73 c axis (angstrom) 4.794.737 Volume (angstrom³) 272.35 264.73 Expansion/Contraction 6.5 7.0 (%)

As shown in Table 2, each of the actually prepared cathode activematerials exhibited a rate of change in volume of 6.5%, which is almostthe same as the calculated value of 7.0%.

(II) Preparation of a Cathode Active Material Example 1

A lithium source Li(OC₂H₅), an iron source Fe(CH₃COO)₂, a zirconiumsource Zr(OC₂H₅)₄, a phosphate source (NH₄)₂HPO₄, and a silicon sourceSi(OC₂H₅)₄ were used as starting materials, and these starting materialswere measured out so that the molar ratio wasLi:Fe:Zr:P:Si=1:0.875:0.125:0.75:0.25. Next, the Li source, the Zrsource, and the Si source were dissolved in 20 g of butanol. Further,the Fe source and the P source were dissolved in water whose number ofmoles was four times as large as that total number of moles of metalalcoxide (the Fe source, the Si source, and the Li source). After onehour of stirring of a mixture of the butanol, in which the metalalcoxide had been dissolved, and the water, in which the Fe source andthe P source had been dissolved, the resulting mixture was dried at 60°C. in a dryer into a powdery precursor.

The resultant amorphous precursor was sintered for twelve hours at 600°C. in a nitrogen atmosphere. Thus synthesized wasLiFe_(0.875)Zr_(0.125)P_(0.75)Si_(0.25)O₄ single-phase powder, which isan olivine-type cathode active material. The lattice constants of theresultant cathode active material along the a axis, the b axis, and thec were 10.144, 6.003, and 4.712, respectively.

Example 2 Preparation of a Cathode Active Material

A lithium source LiCH₃COO, an iron source Fe(NO₃)₃.9H₂O, a zirconiumsource ZrCl₄, a phosphate source H₃PO₄ (85%), and a silicon sourceSi(OC₂H₅)₄ were used as starting materials. These starting materialswere measured out so that the molar ratio isLi:Fe:Zr:P:Si=1:0.75:0.25:0.5:0.5, with the lithium source LiCH₃COO usedin an amount of 1.3196 g. These starting materials were dissolved in 30ml of C₂H₅OH and stirred by a stirrer for 48 hours at room temperature.After that, the solvent was removed at 40° C. in a constant-temperaturebath, with the result that a brownish-red powder was obtained.

After addition of 15 percent by weight of sucrose relative to theresultant powder, they were mixed well in an agate mortar, and theresulting mixture was pressure-molded into pellets. The pellets weresintered for twelve hours at 600° C. in a nitrogen atmosphere. Thussynthesized was LiFe_(0.75)Zr_(0.25)P_(0.5)Si_(0.5)O₄ single-phasepowder. The resultant cathode active material is referred to as “Al”.

<Preparation of a Cathode Electrode>

Approximately 1 gram of the cathode active material A2 obtained as abovewas weighed out, crushed in an agate mortar, and then mixed withapproximately 10 percent by weight of a conductive agent, acetyleneblack (marketed as “Denka Black”; manufactured by Denki Kagaku KogyoKabushiki Kaisha), relative of the cathode active material andapproximately 10 percent by weight of a binding agent, polyvinylidenefluoride resin powder, relative to the cathode active material.

This mixture was dissolved in a solvent such as N-methyl-2-pyrrolidoneto form slurry, and the slurry was applied onto both surfaces of a20-μm-thick aluminum foil by a doctor blade method so that the amount ofapplication was approximately 5 mg/cm². This electrode was dried, andthen cut so that the applied electrode surface was 2 cm×2 cm. Theelectrode was then pressed to provide a cathode electrode.

Example 3

A lithium source LiCH₃COO, an iron source Fe(NO₃)₃.9H₂O, a zirconiumsource ZrCl₄, a phosphate source H₃PO₄ (85%), and a silicon sourceSi(OC₂H₅)₄ were used as starting materials. These starting materialswere measured out so that the molar ratio isLi:Fe:Zr:P:Si=1:0.85:0.15:0.7:0.3, with the lithium source LiCH₃COO usedin an amount of 1.3196 g. These starting materials were dissolved in 30ml of C₂H₅₀H and stirred by a stirrer for 48 hours at room temperature.After that, the solvent was removed at 40° C. in a constant-temperaturebath, with the result that a brownish-red powder was obtained.

After addition of 15 percent by weight of sucrose relative to theresultant powder, they were mixed well in an agate mortar, and theresulting mixture was pressure-molded into pellets. The pellets weresintered for twelve hours at 600° C. in a nitrogen atmosphere. Thussynthesized was LiFe_(0.85) Zr_(0.15)P_(0.7)Si_(0.3)O₄ single-phasepowder. The resultant cathode active material is referred to as “A2”.

The operation performed in Example 2 was performed also on the cathodeactive material A2 to prepare a cathode electrode.

Example 4

A lithium source LiCH₃COO, an iron source Fe(NO₃)₃.9H₂O, a zirconiumsource ZrCl₄, a phosphate source H₃PO₄ (85%), and a silicon sourceSi(OC₂H₅)₄ were used as starting materials. These starting materialswere measured out so that the molar ratio isLi:Fe:Zr:P:Si=1:0.875:0.125:0.75:0.25, with the lithium source LiCH₃COOused in an amount of 1.3196 g. These starting materials were dissolvedin 30 ml of C₂H₅OH and stirred by a stirrer for 48 hours at roomtemperature. After that, the solvent was removed at 40° C. in aconstant-temperature bath, with the result that a brownish-red powderwas obtained.

After addition of 15 percent by weight of sucrose relative to theresultant powder, they were mixed well in an agate mortar, and theresulting mixture was pressure-molded into pellets. The pellets weresintered for twelve hours at 600° C. in a nitrogen atmosphere. Thussynthesized was LiFe_(0.875)Zr_(0.125)P_(0.75)Si_(0.25)O₄ single-phasepowder. The resultant cathode active material is referred to as “A3”.

The operation performed in Example 2 was performed also on the cathodeactive material A3 to prepare a cathode electrode.

Example 5

A lithium source LiCH₃COO, an iron source Fe(NO₃)₃.9H₂O and ZrCl₄, aphosphate source H₃PO₄ (85%), and a silicon source Si(OC₂H₅)₄ were usedas starting materials. These starting materials were measured out sothat the molar ratio is Li:Fe:Zr:P:Si=1:0.9:0.1:0.8:0.2, with thelithium source LiCH₃COO used in an amount of 1.3196 g. These startingmaterials were dissolved in 30 ml of C₂H₅OH and stirred by a stirrer for48 hours at room temperature. After that, the solvent was removed at 40°C. in a constant-temperature bath, with the result that a brownish-redpowder was obtained.

After addition of 15 percent by weight of sucrose relative to theresultant powder, they were mixed well in an agate mortar, and theresulting mixture was pressure-molded into pellets. The pellets weresintered for twelve hours at 600° C. in a nitrogen atmosphere. Thussynthesized was LiFe_(0.9)Zr_(0.1)P_(0.8)Si_(0.2)O₄ single-phase powder.The resultant cathode active material is referred to as “A4”.

The operation performed in Example 2 was performed also on the cathodeactive material A4 to prepare a cathode electrode.

Example 6

A lithium source LiCH₃COO, an iron source Fe(NO₃)₃.9H₂O, a zirconiumsource ZrCl₄, a phosphate source H₃PO₄ (85%), and a silicon sourceSi(OC₂H₅)₄ were used as starting materials. These starting materialswere measured out so that the molar ratio isLi:Fe:Zr:P:Si=1:0.93:0.07:0.86:0.14, with the lithium source LiCH₃COOused in an amount of 1.3196 g. These starting materials were dissolvedin 30 ml of C₂H₅OH and stirred by a stirrer for 48 hours at roomtemperature. After that, the solvent was removed at 40° C. in aconstant-temperature bath, with the result that a brownish-red powderwas obtained.

After addition of 15 percent by weight of sucrose relative to theresultant powder, they were mixed well in an agate mortar, and theresulting mixture was pressure-molded into pellets. The pellets weresintered for twelve hours at 600° C. in a nitrogen atmosphere. Thussynthesized was LiFe_(0.93)Zr_(0.07)P_(0.86)Si_(0.14)O₄ single-phasepowder. The resultant cathode active material is referred to as “A5”.

The operation performed in Example 2 was performed also on the cathodeactive material A5 to prepare a cathode electrode.

Example 7

A lithium source LiCH₃COO, an iron source Fe(NO₃)₃.9H₂O, a zirconiumsource ZrCl₄, a phosphate source H₃PO₄ (85%), and a silicon sourceSi(OC₂H₅)₄ were used as starting materials. These starting materialswere measured out so that the molar ratio isLi:Fe:Zr:P:Si=1:0.95:0.05:0.9:0.1, with the lithium source LiCH₃COO usedin an amount of 1.3196 g. These starting materials were dissolved in 30ml of C₂H₅OH and stirred by a stirrer for 48 hours at room temperature.After that, the solvent was removed at 40° C. in a constant-temperaturebath, with the result that a brownish-red powder was obtained.

After addition of 15 percent by weight of sucrose relative to theresultant powder, they were mixed well in an agate mortar, and theresulting mixture was pressure-molded into pellets. The pellets weresintered for twelve hours at 600° C. in a nitrogen atmosphere. Thussynthesized was LiFe_(0.95)Zr_(0.05)P_(0.9)Si_(0.1)O₄ single-phasepowder. The resultant cathode active material is referred to as “A6”.

The operation performed in Example 2 was performed also on the cathodeactive material A6 to prepare a cathode electrode.

Comparative Example 1

A lithium source LiCH₃COO, an iron source Fe(NO₃)₃. 9H₂O, and aphosphate source H₃PO₄ (85%) were used as starting materials. Thesestarting materials were measured out so that the molar ratio isLi:Fe:P=1:1:1, with the lithium source LiCH₃COO used in an amount of1.3196 g. These starting materials were dissolved in 30 ml of C₂H₅OH andstirred by a stirrer for 48 hours at room temperature. After that, thesolvent was removed at 40° C. in a constant-temperature bath, with theresult that a brownish-red powder was obtained.

After addition of 15 percent by weight of sucrose relative to theresultant powder, they were mixed well in an agate mortar, and theresulting mixture was pressure-molded into pellets. The pellets weresintered for twelve hours at 600° C. in a nitrogen atmosphere. Thussynthesized was a cathode active material. The resultant cathode activematerial is referred to as “B1”.

The operation performed in Example 2 was performed also on the cathodeactive material B1 to prepare a cathode electrode.

(III) Evaluation of Cathode Active Material

(III-I) X-Ray Analysis

The cathode active materials A1 to A6 and B1 thus obtained were eachcrushed in an agate mortar and subjected to a X-ray analysis apparatus(marketed as MiniFlexII; manufactured by Rigaku Co., Ltd.) to give apowder X-ray diffraction pattern. FIGS. 1 through 7 show X-raydiffraction patterns for the cathode active materials A1 to A6 and B1,respectively, as the results of the X-ray analysis.

(III-II) Evaluation of Valence of Fe

The respective cathode active materials prepared in the Examples andComparative Example were each crushed in an agate mortar and subjectedto a Mössbauer spectrometry with use of a Mössbauer spectroscopy device.

A Mössbauer absorption spectrum was measured under the followingconditions: A gamma ray source was ⁵⁷Co, which is an isotope of cobalt.A sample targeted for the measurement was placed in an amount of 200 mgbetween the gamma ray source and a gamma ray detector. The sample wasvibrated at an amplitude of 5 cm±6 mm/s with respect to the detector. AMössbauer spectrum was measured by measuring absorption of gamma rays.

Assuming that four absorption peaks centered at the respective velocityregions of −0.1 mm/s, 0 mm/s, 1 mm/s, and 2.7 mm/s were Lorentzfunctions, fittng was performed by a least-squares method on the dataobtained as above. The respective peaks at the velocity regions of −0.1mm/s and 2.7 mm/s were presumed to be due to absorption by Fe²⁺, and therespective peaks at the velocity regions of 0 mm/s and 1 mm/s werepresumed to be due to absorption by Fe³⁺. A ratio between Fe²⁺ and Fe³⁺was calculated from an area ratio of the respective peaks for Fe²⁺ andFe³⁺.

FIG. 8 shows an absorption spectrum, measured by the above method, ofthe cathode active material A1. This spectrum measurement result showsthat (i) two large absorption peaks were observed, that (ii) a value ofan isomer shift, that is, a medium value between the two peaks, wasapproximately 1.25, and that (iii) a quadropole split, corresponding toa distance between the peaks, was approximately 2.8. The aboveabsorption peaks coincide well with typical absorption peaks of Fe²⁺.The spectrum of the cathode active material A1 showed, other than thepeaks attributed to Fe²⁺, peaks attributed to Fe³⁺, the peaks having anisomer shift value of approximately 0.5 and a quadropole split ofapproximately 0.6 to 1.0. These results showed that the cathode activematerial A1 was made up of Fe²⁺ and Fe³⁺. An area ratio between Fe²⁺ andFe³⁺ in the above spectrum showed that Fe²⁺:Fe³⁺=95:5.

FIG. 9 shows an absorption spectrum obtained by a Mössbauer spectrometryfor the cathode active material A2. This spectrometry result shows that(i) two large absorption peaks were observed, and as a result of fittingthe peaks by double Lorentzian, that (ii) a value of an isomer shift,that is, a medium value between the two peaks, was approximately 1.25,and that (iii) a quadropole split, corresponding to a distance betweenthe peaks, was approximately 2.8. The above absorption peaks coincidewell with typical absorption peaks of Fe²⁺. This showed that the cathodeactive material A2 was made up of Fe²⁺. It should be noted that no peakattributed to Fe³⁺ was observed, unlike in the cathode active materialA1.

A measurement similar to the above was performed on each of the othercathode active materials A3 to A6, with a result similar to the above.This confirmed that the iron in each of the cathode active materials A2to A6 had a valence of 2⁺.

(IV) Evaluation of Battery

(IV-I) Capacity Ratio

Put into a 50-ml beaker a 30 ml electrolyte. The electrolyte was mixed50% by volume of diethyl carbonate with 50% by volume of ethylenecarbonate. 1 mol/L of LiPF₆ was dissolved in the electrolyte. With useof (i) the cathode electrode prepared in each of the Examples and theComparative Example and (ii) metal lithium as an anodic active materialserving as a counter electrode, a battery was prepared.

Each of the batteries thus prepared was first charged in an environmentof 25° C. The charging current was 0.1 mA, and the charging was finishedat a point in time where the battery reached a potential of 4V. Afterthe charging was finished, the battery was discharged at 0.1 mA, and thedischarging was finished at a point in time where the battery reached apotential of 2.0 V, with the result that the actually measured capacityof the battery was obtained. These results are shown in Table 3.

TABLE 3 Actually Cathode Theoretical measured Capacity active capacitycapacity ratio material (mAh/g) (mAh/g) (%) Example 2 A1 121.7 110.490.6% Example 3 A2 140.5 128.4 91.4% Example 4 A3 145.3 130.3 89.7%Example 5 A4 150.1 127.9 85.2% Example 6 A5 156.0 138.0 88.5% Example 7A6 159.9 140.3 87.7% Comparative B1 169.9 147.5 86.8% Example 1

(IV-II) Rate of Change in Volume

Furthermore, each battery prepared in “(IV-I) Capacity Ratio” wascharged at a constant current of 0.1 mA until 4 V so that lithium wasdesorbed. After that, the lattice constant after lithium desorption wascalculated by taking out the electrode and performing powder X-raydiffractometry on the electrode.

Table 4 shows lattice constants before charging. Table 5 shows latticeconstants after charging. Table 6 shows rates of change in volume.

TABLE 4 Cathode Lattice constant Lattice active a axis b axis c axisvolume material (angstrom) (angstrom) (angstrom) (angstrom³) Example 2A1 10.413 6.031 4.750 298.3 Example 3 A2 10.366 6.022 4.715 294.3Example 4 A3 10.355 6.020 4.712 293.7 Example 5 A4 10.343 6.010 4.706292.5 Example 6 A5 10.335 6.005 4.701 291.8 Example 7 A6 10.332 6.0054.699 291.6 Comparative B1 10.328 6.007 4.696 291.3 Example 1

TABLE 5 Cathode Lattice constant Lattice active a axis b axis c axisvolume material (angstrom) (angstrom) (angstrom) (angstrom³) Example 2A1 10.190 6.015 4.877 298.9 Example 3 A2 10.077 5.934 4.819 288.2Example 4 A3 9.997 5.884 4.808 282.8 Example 5 A4 9.972 5.862 4.796280.4 Example 6 A5 9.948 5.852 4.792 279.0 Example 7 A6 9.912 5.8404.790 277.3 Comparative B1 9.830 5.802 4.785 272.9 Example 1

TABLE 6 Rate of change Cathode in active volume material (%) Example 2A1 −0.2   Example 3 A2 2.1 Example 4 A3 3.7 Example 5 A4 4.2 Example 6A5 4.4 Example 7 A6 4.9 Comparative B1 6.3 Example 1

(IV-III) Evaluation of Capacity Retention Rate

<Preparation of Battery>

Used as an anodic active material were natural graphite powder andlithium titanate (Li₄Ti₅O₁₂). The anodic active material was mixed withapproximately 10% by weight of polyvinylidene fluoride resin powderserving as a binding agent. Further, in the case where lithium titanatewas used as an anodic active material, 10% by weight of acetylene blackwas mixed as a conductive agent. This mixture was dissolved inN-methyl-2-pyrrolidone to form slurry, and the slurry was applied ontoboth surfaces of a 20-μm-thick copper foil. The applied slurry was driedand then pressed to provide an anode.

The cathode prepared in each of the Examples and the Comparative Exampleand the above anode were each cut out in a size of 30 mm×30 mm. As acurrent introducing terminal for a battery, an aluminum tab having awidth of 3 mm and a length of 50 mm was welded to the cathode, whereas acopper tab having a width of 3 mm and a length of 50 mm was welded tothe anode. Thus prepared were a cathode electrode and an anodeelectrode.

A separator made of porous polyethylene was placed between the cathodeelectrode and the anode electrode. The layered product thus prepared wasplaced between laminate films including two metal foils to each of whicha thermoplastic resin was attached. The metal foils were thermallywelded to each other along the periphery to be sealed, which providedthe battery with an armor. This laminate had an opening for injecting anelectrolyte.

In the laminate, 50% by volume of ethylene carbonate, in which 1 mol/Lof LiPF₆ was dissolved, and 50% by volume of diethyl carbonate wereimpregnated as an electrolyte.

After the electrolyte was injected in the battery, the opening of thebattery container was sealed, to complete the preparation of a secondarybattery.

FIG. 10 is a cross-sectional view illustrating the battery prepared asabove. FIG. 10 illustrates a cathode electrode 1, an anode electrode 2,a separator 3, cathode and anode tabs 4, and a laminate 5.

<Evaluation of Capacity Retention Rate>

Each of the batteries thus prepared was first charged in an environmentof 25° C. The charging current was 0.2 mA, and the charging was finishedat a point in time where the battery reached a potential of 4V. Afterthe charging was finished, the battery was discharged at 0.2 mA, and thedischarging was finished at a point in time where the battery reached apotential of 2.0 V, with the result that the initial capacity of thebattery was obtained. Further, the battery was repeatedly charged anddischarged at a current of 0.2 mA. A discharging capacity of the batteryat a hundredth cycle was then measured, and a capacity retention ratewas calculated according to the following formula:Capacity retention rate=discharging capacity at hundredth cycle/initialdischarging capacity.

Table 7 shows the results for the case where the anode is made ofcarbon. Table 8 shows the results for the case where the anode is madeof lithium titanate.

TABLE 7 Initial Discharging Capacity Cathode discharging capacity atretention active capacity hundredth rate material (mAh/g) cycle (mAh/g)(%) Example 2 A1 102.1 100.9 98.8 Example 3 A2 118.9 117.8 99.1 Example4 A3 118.6 115.6 97.5 Example 5 A4 122.1 120.9 99.0 Example 6 A5 128.4123.5 96.2 Example 7 A6 131.5 125.8 95.7 Comparative B1 136.4 110.5 81.0Example 1

TABLE 8 Initial Discharging Capacity Cathode discharging capacity atretention active capacity hundredth rate material (mAh/g) cycle (mAh/g)(%) Example 2 A1 108.0 105.9 98.0 Example 3 A2 125.2 123.3 98.4 Example4 A3 126.1 123.5 97.9 Example 5 A4 127.8 125.7 98.3 Example 6 A5 136.5129.8 95.1 Example 7 A6 138.3 130.8 94.6 Comparative B1 145.6 130.8 89.8Example 1

Tables 7 and 8 show the following: The batteries including therespective cathode active materials A 1 to A6 are superior in capacityitself and capacity retention rate to the battery including the cathodeactive material B1 of Comparative Example 1. Among the batteriesincluding the respective cathode active materials A1 to A6, thebatteries including the respective cathode active materials A2 to A6, ineach of which every Fe has a valence of +2, are superior in property tothe battery including the cathode active material A1, which includes Featoms having a valence of 3.

Further, the batteries including the respective cathode active materialsA1 to A3 each have a capacity retention rate of approximately 99%, andare thus extremely excellent in cycling characteristics.

The batteries including the respective cathode active materials A4 to A6are, on the other hand, lower in capacity retention rate than thebatteries including the respective cathode active materials A 1 to A3,but are better in cycling characteristics than the cathode activematerial of Comparative Example 1 and also larger in capacity itselfthan the batteries including the respective cathode active materials A 1to A3. Thus, an application that requires a long life preferablyinvolves as a cathode active material a composition, such as the cathodeactive materials A1 to A3, in which the amount of substitution is0.1≦x≦0.5, whereas an application that requires a long life and a highercapacity preferably involves as a cathode active material composition,such as the cathode active materials A4 to A6, in which the amount ofsubstitution is 0.05≦x≦0.1.

(IV-IV) Evaluation of Changes in Thickness During Charging andDischarging

Example 8

Ten grams of the cathode active material A1 obtained in Example 1 wereweighed out, crushed in an agate mortar, and then mixed withapproximately 10 percent by weight of a conductive agent, acetyleneblack (marketed as “Denka Black”; manufactured by Denki Kagaku KogyoKabushiki Kaisha), relative of the cathode active material andapproximately 10 percent by weight of a binding agent, polyvinylidenefluoride resin powder, relative to the cathode active material.

This mixture was dissolved in a solvent such as N-methyl-2-pyrrolidoneto form slurry, and the slurry was applied onto both surfaces of a20-μm-thick aluminum foil by a doctor blade method so that the amount ofapplication was approximately 20 mg/cm². This electrode was dried, andthen oil-pressed so that its thickness was approximately 100 μm,including the thickness of the aluminum foil. Thus electrode was anelectrode having an electrode size of 2 cm×2 cm.

After the electrode had been dried, a battery was prepared by using theelectrode as a cathode, using Li metal as a counter electrode, andpouring 50 ml of an electrolyte into a 100-ml glass container. Theelectrolyte (manufactured by Kishida Chemical Co., Ltd.) used wasobtained by dissolving LiPF₆ in a solvent so that the concentration was1.4 mol/l, and the solvent used was obtained by mixing ethylenecarbonate and diethyl carbonate with a volume ratio of 7:3.

As a result of charging of the resultant battery at 0.1 mA, a chargingcapacity of 140 mAh/g was obtained. As a result of measurement of thethickness of the cathode taken out after completion of charging, thecathode had a thickness of 97 μm, while it had had a thickness of 101 μmbefore the charging.

Comparative Example 2

An electrode was prepared through the same procedure as in Example 8except that the cathode active material B1 prepared in ComparativeExample 1 was used instead of the cathode active material A1. A batteryprepared by using the electrode as cathode was charged and discharged,and the thickness of the cathode was measured. As a result, the cathodehad a thickness of 93 μm, while it had had a thickness of 102 μm beforethe charging.

Comparison between Example 8 and Comparative Example 2 shows that acathode according to the present invention has a smaller amount ofchange in thickness during charging and discharging than a conventionalcathode.

Example 9 Flat-Plate Laminate Battery

A lithium source LiCH₃COO, an iron source Fe(NO₃)₃.9H₂O, a zirconiumsource ZrCl₄, a phosphate source H₃PO₄ (85%), and a silicon sourceSi(OC₂H₅)₄ were used as starting materials. These starting materialswere measured out so that the molar ratio isLi:Fe:Zr:P:Si=1:0.75:0.25:0.5:0.5, with the lithium source LiCH₃COO usedin an amount of 131.96 g. These starting materials were dissolved in3000 ml of C₂H₅OH and stirred by a stirring motor for 48 hours at roomtemperature. After that, the solvent was removed at 40° C. in aconstant-temperature bath, with the result that a brownish-red powderwas obtained.

Two hundred grams of the resultant brownish-red powder were weighed out,crushed in steps of 10 g in an automatic mortar, and then mixed withapproximately 10 percent by weight of a conductive agent, acetyleneblack (marketed as “Denka Black”; manufactured by Denki Kagaku KogyoKabushiki Kaisha), relative of the cathode active material andapproximately 10 percent by weight of binding agent, polyvinylidenefluoride resin powder, relative to the cathode active material.

This mixture was dissolved in a solvent such as N-methyl-2-pyrrolidoneto form slurry, and the slurry was applied onto both surfaces of a20-μm-thick aluminum foil by a doctor blade method. After the slurry hadbeen applied onto one surface, the same slurry was applied onto theother surface, whereby an electrode as formed on both surfaces of themetal foil. It should be noted that the slurry was applied so that theamount of application per surface was approximately 15 mg/cm².

After the electrode had been dried, a cathode electrode was prepared bypressing the electrode by passing it through a space between two metalrollers placed at a distance of approximately 130 μm, in order that itsthickness was approximately 150 μm, including the thickness of thealuminum foil.

Next, approximately 500 g of natural graphite powder having an averageparticle diameter of approximately 5 μm were weight out as an anodicactive material, and this anodic active material was mixed withapproximately 10 percent by weight of a binding agent, polyvinylidenefluoride resin powder, relative to the anodic active material.

This mixture was dissolved in a solvent such as N-methyl-2-pyrrolidoneto form slurry, and the slurry was applied onto both surfaces of a12-μm-thick copper foil by a doctor blade method. After the slurry hadbeen applied onto one surface, the same slurry was applied onto theother surface, whereby an electrode as formed on both surfaces of themetal foil. It should be noted that the amount of application persurface was approximately 7 mg/cm².

After the electrode had been dried, an anodic electrode was prepared bypressing the electrode by passing it through a space between two metalrollers placed at a distance of approximately 120 μm, in order that itsthickness was approximately 140 μm, including the thickness of thecopper foil.

The cathode electrode thus obtained was cut into ten cathode electrodeseach having a width of 10 cm and a height of 15 cm. Similarly, theanodic electrode was cut into eleven anodic electrodes each having awidth of 10.6 cm and a height of 15.6 cm. It should be noted that thecathodes and the anodes had their shorter sides provided with unpaintedparts each having a width of 10 mm and a length of 25 mm, and theseunpainted parts served as collector tabs.

As separators, twenty polypropylene porous films each having a thicknessof 25 μm, a width of 11 cm, and a height of 16 cm were used. Such alayered product 11 as shown in FIG. 11 was obtained layering thecathodes, the anodes, and the separators in such a way that theseparators are disposed on both surfaces of the cathodes so that theanodes and the cathodes do not have direct contact with each other; andfixing them with an adhesive tape made of Kapton resin. Weldedultrasonically to each of the cathode tabs of the layered product 11 wasa cathode collector lead 13, made of aluminum, which had a width of 10mm, a length of 30 mm, and a thickness of 100 μm. Similarly weldedultrasonically to each of the anode tabs was an anode collector lead 14,made of nickel, which had a width of 10 mm, a length of 30 mm, and athickness of 100 μm.

The layered product 11 thus prepared was placed between two aluminumlaminates 12, three of whose sides were heat-sealed. In this state, thelayered product 11 was dehydrated by heating it for twelve hours at atemperature of approximately 80° C. in a chamber decompressed by arotary pump.

The layered product 11 thus dried was placed in a dry box in an Aratmosphere, and a flat-plate laminate battery was prepared by injectingapproximately 50 cc of an electrolyte (manufactured by Kishida ChemicalCo., Ltd.) and sealing the opening under reduced pressure. Theelectrolyte used was obtained by dissolving LiPF₆ in a solvent so thatthe concentration was 1.4 mol/l, and the solvent used was obtained bymixing ethylene carbonate and diethyl carbonate with a volume ratio of7:3.

The prepared battery had a thickness of 4.1 mm. A current of 100 mA wasapplied to this battery, and the charging was finished at a point intime where the battery reached a voltage of 3.9 V. After the charging,the battery had a measured thickness of 4.2 mm. This shows that therewas almost no change in thickness during the charging.

Comparative Example 3

A flat-plate laminate battery was prepared through exactly the sameprocedure as in Example 8 except that a lithium source LiCH₃COO, an ironsource Fe(NO₃)₃.9H₂O, and a phosphate source H₃PO₄ (85%) were used asstarting materials and that these starting materials were measured outso that the molar ratio is Li:Fe:P=1:1:1, with the lithium sourceLiCH₃COO used in an amount of 131.96 g.

The prepared battery had a thickness of 4.1 mm. A current of 100 mA wasapplied to this battery, and the charging was finished at a point intime where the battery reached a voltage of 3.9 V. After the charging,the battery had a measured thickness of 4.7 mm.

The results of Example 9 and Comparative Example 3 show that a batteryin which a cathode according to the present invention is used changesless in thickness than a battery in which a conventional cathode isused.

Example 10 Layered Cuboidal Battery

A lithium source LiCH₃COO, an iron source Fe(NO₃)₃.9H₂O, a zirconiumsource ZrCl₄, a phosphate source H₃PO₄ (85%), and a silicon sourceSi(OC₂H₅)₄ were used as starting materials. These starting materialswere measured out so that the molar ratio isLi:Fe:Zr:P:Si=1:0.75:0.25:0.5:0.5, with the lithium source LiCH₃COO usedin an amount of 1319.6 g. These starting materials were dissolved in 30L of C₂H₅OH and stirred by a stirring motor for 48 hours at roomtemperature. After that, the solvent was removed at 40° C. in aconstant-temperature bath, with the result that a brownish-red powderwas obtained.

One thousand grams of the resultant brownish-red powder were weighedout, crushed in steps of 10 g in an automatic mortar, and then mixedwith approximately 10 percent by weight of a conductive agent, acetyleneblack (marketed as “Denka Black”; manufactured by Denki Kagaku KogyoKabushiki Kaisha), relative of the cathode active material andapproximately 10 percent by weight of a binding agent, polyvinylidenefluoride resin powder, relative to the cathode active material.

This mixture was dissolved in a solvent such as N-methyl-2-pyrrolidoneto form slurry, and the slurry was applied onto both surfaces of a20-μm-thick aluminum foil by a doctor blade method. After the slurry hadbeen applied onto one surface, the same slurry was applied onto theother surface, whereby an electrode as formed on both surfaces of themetal foil. It should be noted that that the amount of application persurface was approximately 15 mg/cm².

After the electrode had been dried, a cathode electrode was prepared bypressing the electrode by passing it through a space between two metalrollers placed at a distance of approximately 130 μm, in order that itsthickness was approximately 150 μm, including the thickness of thealuminum foil.

Next, approximately 500 g of natural graphite powder having an averageparticle diameter of approximately 5 μm were weight out as an anodicactive material, and this anodic active material was mixed withapproximately 10 percent by weight of a binding agent, polyvinylidenefluoride resin powder, relative to the anodic active material.

This mixture was dissolved in a solvent such as N-methyl-2-pyrrolidoneto form slurry, and the slurry was applied onto both surfaces of a12-μm-thick copper foil by a doctor blade method. After the slurry hadbeen applied onto one surface, the same slurry was applied onto theother surface, whereby an electrode as formed on both surfaces of themetal foil. It should be noted that the amount of application persurface was approximately 7 mg/cm².

After the electrode had been dried, an anodic electrode was prepared bypressing the electrode by passing it through a space between two metalrollers placed at a distance of approximately 120 μm, in order that itsthickness was approximately 140 μm, including the thickness of thecopper foil.

The cathode electrode thus obtained was cut into ten cathode electrodeseach having a width of 10 cm and a height of 15 cm. Similarly, theanodic electrode was cut into eleven anodic electrodes each having awidth of 10.6 cm and a height of 15.6 cm. It should be noted that thecathodes and the anodes had their shorter sides provided with unpaintedparts each having a width of 10 mm and a length of 25 mm, and theseunpainted parts served as collector tabs.

As separators, twenty polypropylene porous films each processed to havea thickness of 25 μm, a width of 11 cm, and a height of 16 cm were used.

Such a layered product 15 as shown in FIG. 12 was obtained by: layeringthe cathodes, the anodes, and the separators in such a way that theseparators are disposed on both surfaces of the cathodes so that theanodes and the cathodes do not have direct contact with each other; andfixing them with an adhesive tape made of Kapton resin.

Welded ultrasonically to each of the cathode tabs of the layered product15 was a cathode collector lead 16, made of aluminum, which had a widthof 10 mm, a length of mm, and a thickness of 100 μm. Similarly weldedultrasonically to each of the anode tabs was an anode collector lead 17,made of nickel, which had a width of 10 mm, a length of 30 mm, and athickness of 100 μm.

The layered product 15 was dehydrated by heating it for twelve hours ata temperature of approximately 80° C. in a chamber decompressed by arotary pump.

The layered product 15 thus dried was inserted into a battery can 18 ina dry box in an Ar atmosphere, and the collector leads 16 and 17 of thelayered product 15 were welded ultrasonically to the ends of collectorterminals (cathode terminals, anode terminals 21) of a battery lid 19provided with two piercing terminals and made of an aluminum metalplate. It should be noted that the battery can 18 used was a 1-mm-thickaluminum can shaped into cuboid with the dimensions 12 cm×18 cm×2 cm andprovided with a safety valve 20.

Then, the battery lid 19 was fitted in the opening of the battery can18, and the battery was sealed by laser-welding the joint.

A cuboidal battery was prepared by injecting approximately 300 cc of anelectrolyte (manufactured by Kishida Chemical Co., Ltd.) through a holeof 1 mm diameter made in the battery lid 19 and then sealing theinjection hole by laser welding. The electrolyte used was obtained bydissolving LiPF₆ in a solvent so that the concentration was 1.4 mol/l,and the solvent used was obtained by mixing ethylene carbonate anddiethyl carbonate with a volume ratio of 7:3.

The prepared battery had a thickness of 20.0 mm in its central part. Acurrent of 100 mA was applied to this battery, and the charging wasfinished at a point in time where the battery reached a voltage of 3.9V. After the charging, the battery had a measured thickness of 20.0 mmin its central part. This shows that there was almost no change inthickness during the charging.

Comparative Example 4

A layered cuboidal battery was prepared through exactly the sameprocedure as in Example 10 except that a lithium source LiCH₃COO, aniron source Fe(NO₃)₃.9H₂O, and a phosphate source H₃PO₄ (85%) were usedas starting materials and that these starting materials were measuredout so that the molar ratio is Li:Fe:P=1:1:1, with the lithium sourceLiCH₃COO used in an amount of 131.96 g.

The prepared battery had a thickness of 20.0 mm in its central part. Acurrent of 100 mA was applied to this battery, and the charging wasfinished at a point in time where the battery reached a voltage of 3.9V. After the charging, the battery had a measured thickness of 21.5 mmin its central part.

The results of Example 10 and Comparative Example 4 show that a batteryin which a cathode according to the present invention is used changesless in thickness than a battery in which a conventional cathode isused.

(IV-V) Evaluation of the Capacity Retention Rate of Wound CylindricalBattery

Example 11 Wound Cylindrical Battery

A lithium source LiCH₃COO, an iron source Fe(NO₃)₃.9H₂O, a zirconiumsource ZrCl₄, a phosphate source H₃PO₄ (85%), and a silicon sourceSi(OC₂H₅)₄ were used as starting materials. These starting materialswere measured out so that the molar ratio isLi:Fe:Zr:P:Si=1:0.75:0.25:0.5:0.5, with the lithium source LiCH₃COO usedin an amount of 1319.6 g. These starting materials were dissolved in 30L of C₂H₅OH and stirred by a stirring motor for 48 hours at roomtemperature. After that, the solvent was removed at 40° C. in aconstant-temperature bath, with the result that a brownish-red powderwas obtained.

One thousand grams of the resultant brownish-red powder were weighedout, crushed in steps of 10 g in an automatic mortar, and then mixedwith approximately 10 percent by weight of a conductive agent, acetyleneblack (marketed as “Denka Black”; manufactured by Denki Kagaku KogyoKabushiki Kaisha), relative of the cathode active material andapproximately 10 percent by weight of a binding agent, polyvinylidenefluoride resin powder, relative to the cathode active material.

This mixture was dissolved in a solvent such as N-methyl-2-pyrrolidoneto form slurry, and the slurry was applied onto both surfaces of a20-μm-thick aluminum foil by a doctor blade method. After the slurry hadbeen applied onto one surface, the same slurry was applied onto theother surface, whereby an electrode as formed on both surfaces of themetal foil. It should be noted that that the amount of application persurface was approximately 15 mg/cm².

After the electrode had been dried, a cathode electrode was prepared bypressing the electrode by passing it through a space between two metalrollers placed at a distance of approximately 130 μm, in order that itsthickness was approximately 150 μm, including the thickness of thealuminum foil.

Next, approximately 500 g of natural graphite powder having an averageparticle diameter of approximately 5 μm were weight out as an anodicactive material, and this anodic active material was mixed withapproximately 10 percent by weight of a binding agent, polyvinylidenefluoride resin powder, relative to the anodic active material.

This mixture was dissolved in a solvent such as N-methyl-2-pyrrolidoneto form slurry, and the slurry was applied onto both surfaces of a12-μm-thick copper foil by a doctor blade method. After the slurry hadbeen applied onto one surface, the same slurry was applied onto theother surface, whereby an electrode as formed on both surfaces of themetal foil. It should be noted that the amount of application persurface was approximately 7 mg/cm².

After the electrode had been dried, an anodic electrode was prepared bypressing the electrode by passing it through a space between two metalrollers placed at a distance of approximately 120 μm, in order that itsthickness was approximately 140 μm, including the thickness of thecopper foil.

The cathode electrode thus obtained was cut so as to have a width of 5cm and a length of 150 cm. Similarly, the anodic electrode was cut so asto have a width of 5.2 cm and a height of 160 cm.

The cathodes and the anodes had their shorter sides provided withunpainted parts to which collector tabs were welded. Weldedultrasonically to each of the unpainted parts was a metal lead having awidth of 4 mm, a thickness of 100 μm, and a length of 10 cm. Further, asthose metal leads for the cathodes were made of aluminum, and those forthe anodes were made of nickel.

As a separator, a 25-μm-thick polypropylene porous film processed tohave a width of 6 cm and a length of 350 cm was used. The separator wasfolded in half so as to have a width of 6 cm and a length of 175 cm, andthe cathode was sandwiched between the halves. Such a cylindrical woundproduct 22 as shown in FIG. 13 was obtained by putting the anode on topof the intermediate product and winding it around a polyethylene spindlehaving a diameter of 5 mm and a length of 6.5 cm. The final woundproduct 22 was bound tightly with a Kapton tape so as not to be unwound.

The wound product 22 thus prepared was dehydrated by heating it fortwelve hours at a temperature of approximately 80° C. in a chamberdecompressed by a rotary pump. It should be noted that after thisoperation, following operations were carried out in an argon dry box ata dew point of −40° C. or lower.

An aluminum pipe, having a diameter of 30 mm and a length of 70 mm, oneend of which had been closed by welding an aluminum disc having adiameter of 30 cm was used as a battery can 24. It should be noted thata bottom lid was joined by laser welding.

The wound product 22 was inserted into the battery can 24 and, as shownin FIG. 13, a cathode collector lead 23 was spot-welded to a cathodeterminal 25 of a battery lid 26, and an anode lead (not shown) wasspot-welded to the bottom surface of the battery can 24. Then, thebattery was sealed by laser-welding the battery lid 26 to the opening ofthe cylinder.

Then, a cylindrical battery was prepared by injecting approximately 5 ccof an electrolyte (manufactured by Kishida Chemical Co., Ltd.) through ahole of 1 mm diameter made in the battery lid 26 and then sealing theinjection hole by laser welding. The electrolyte used was obtained bydissolving LiPF₆ in a solvent so that the concentration was 1.4 mol/l,and the solvent used was obtained by mixing ethylene carbonate anddiethyl carbonate with a volume ratio of 7:3.

Five such batteries were prepared. A current of 100 mA was applied toeach of the batteries, and the charging was finished at a point in timewhere the battery reached a voltage of 3.9V and, furthermore, thebattery was discharged until 2.2V. This cycle was repeated a hundredtimes. Table 9 shows the result of an evaluation.

Comparative Example 5

A cylindrical battery was prepared through exactly the same procedure asin Example 11 except that a lithium source LiCH₃COO, an iron sourceFe(NO₃)₃.9H₂O, and a phosphate source H₃PO₄ (85%) were used as startingmaterials and that these starting materials were measured out so thatthe molar ratio is Li:Fe:P=1:1:1, with the lithium source LiCH₃COO usedin an amount of 131.96 g.

Table 9 shows the result of a charge-discharge evaluation carried outthrough exactly the same procedure as in Example 11. As shown in Table9, it was confirmed that the battery of the present invention has ahigher capacity retention ratio and a longer life than the comparativeexample.

TABLE 9 Initial Discharging discharging capacity at Capacity capacityhundredth retention n (mAh/g) cycle (mAh/g) rate (%) Example 11 1 2.612.58 98.7 2 2.62 2.60 99.2 3 2.60 2.59 99.5 4 2.66 2.66 100 5 2.64 2.6198.9 Comparative 1 3.02 2.88 95.2 Example 5 2 3.11 3.00 96.5 3 3.03 2.9497.2 4 3.04 2.83 93.2 5 3.00 2.83 94.5

INDUSTRIAL APPLICABILITY

A cathode active material of the present invention not only excels interms of safety and cost but also can provide a long-life battery, andas such, can be suitably used as a cathode active material in anonaqueous secondary battery such as a lithium ion battery.

REFERENCE SIGNS LIST

-   -   11, 15 layered product    -   12 aluminum laminate    -   13, 16, 23 cathode collector lead    -   14, 17 anode collector lead    -   18, 24 battery can    -   20 safety valve    -   21 anode terminal    -   22 wound product    -   25 cathode terminal

The invention claimed is:
 1. A cathode active material having acomposition represented by General Formula (1) below,LiFe_(1-x)M_(x)P_(1-y)Si_(y)O₄  (1), where: an average valence of Fe is+2 or more; M is an element having a valence of +2 or more and is atleast one type of element selected from a group consisting of Zr, Y, andAl; the valence of M is different from the average valence of Fe;0<x≦0.5; and 0<y<1, wherein: a rate of chane in volume of a unit cell inLi_(x)Fe_(1-x)M_(x)P_(1-y)Si_(y)O₄ is 4% or less with resect to a volumeof a unit cell in General Formula (1).
 2. The cathode active materialaccording to claim 1, wherein:y=x×({valence of M}−2)+(1−x)×({average valence of Fe}−2).
 3. The cathodeactive material according to claim 1, wherein: the valence of M is +4.4. The cathode active material according to claim 3, wherein: M inGeneral Formula (1) is Zr; and 0.15≦x≦0.5.
 5. The cathode activematerial according to claim 3, wherein: M in General Formula (1) is Zr;and 0.25≦x≦0.5.
 6. The cathode active material according to claim 1,wherein: the valence of M in General Formula (1) is +3.
 7. The cathodeactive material according to claim 6, wherein: M in General Formula (1)is Y; and 0.2≦x≦0.5.
 8. The cathode active material according to claim6, wherein: M in General Formula (1) is Al; and 0.35≦x≦0.5.
 9. Thecathode active material according to claim 1, wherein: the averagevalence of Fe in General Formula (1) is +2.
 10. The cathode activematerial according to claim 9, wherein: M in General Formula (1) is Zr;and 0.05≦x≦0.15.
 11. A cathode comprising: the cathode active materialaccording to claim 1; a conductive material; and a binder.
 12. Anonaqueous secondary battery, comprising: the cathode according to claim11; an anode; an electrolyte; and a separator.
 13. The nonaqueoussecondary battery according to claim 12, when the battery reached avoltage of not less than 3.9 V, the cathode active material has acomposition represented by General Formula (2) below,Li_(x)Fe_(1-x)M_(x)P_(1-y)Si_(y)O₄  (2).
 14. The nonaqueous secondarybattery according to claim 13, wherein: the nonaqueous secondary batteryis one of a laminate battery, a layered cuboidal battery, a woundcuboidal battery, and a wound cylindrical battery.
 15. The nonaqueoussecondary battery according to claim 13, wherein: the nonaqueoussecondary battery has an armor of the battery in the case where thearmor is made of a metal.
 16. A module comprising: a combination of aplurality of the nonaqueous secondary battery according to claim
 12. 17.A power storage system comprising: the nonaqueous secondary batteryaccording to claim
 12. 18. The power storage system according to claim17, wherein: the power storage system is one of a power storage systemfor a solar battery, a storage system for late-night power, a powerstorage system for wind power generation, a power storage system forgeothermal power generation, and a power storage system for waveactivated power generation.